The present invention relates to techniques for aligning a subject in a space coordinate system, and in particular, an apparatus and a method that employ on a subject at least one holographic optical element for developing two beams of focused light whose focal configurations appear in predetermined locations which correspond to the position of the subject in the system.
Systems of various types often require that a component included therein be oriented in an initial position, displaced from the initial position or removed from the system, and later returned to the initial position in the system. U.S. Pat. No. 4,516,833 of Fusek exemplifies an optical system of this type.
In the system of Fusek, a photographic plate positioned in the Fourier transform plane is exposed with light of a predetermined intensity diffracted by a photomask. The photographic plate, which comprises a flat glass substrate, is supported by a holder in the optical system. Having been exposed to the diffracted light rays, the glass plate is removed from the holder, photographically developed returned to the holder, and aligned in exactly the position it occupied during exposure. A hologram of the photomask is recorded from the light rays passing through the plate, which functions as an optical filter. The diffracted light rays must strike the same locations on the plate; therefore, the plate must occupy the same position it held during exposure.
To meet the critical alignment requirements for fulfilling this objective, the holder must be secured in a stationary position in the optical system and be designed so that the plate can be returned to the same position it occupied during exposure. Relying on close tolerances in the fabrication of the plate and holder is insufficient for preventing misalignment that results from, for example, a chip present along a side margin of the glass plate.